light reflex

reflex

[re´fleks]

a reflected action or movement; the sum total of any particular automatic response mediated by the nervous system. A reflex is built into the nervous system and does not need the intervention of conscious thought to take effect.

The knee jerk is an example of the simplest type of reflex. When the knee is tapped, the nerve that receives this stimulus sends an impulse to the spinal cord, where it is relayed to a motor nerve. This causes the quadriceps muscle at the front of the thigh to contract and jerk the leg up. This reflex, or simple reflex arc, involves only two nerves and one synapse. The leg begins to jerk up while the brain is just becoming aware of the tap.

Other simple reflexes, the stretch reflexes, help the body maintain its balance. Every time a muscle is stretched, it reacts with a reflex impulse to contract. As a person reaches or leans, the skeletal muscles tense and tighten, tending to hold him and keep him from falling. Even in standing still, the stretch reflexes in the skeletal muscles make many tiny adjustments to keep the body erect.

The “hot stove” reflex is more complex, calling into play many different muscles. Before the hand is pulled away, an impulse must go from the sensory nerve endings in the skin to a center in the spinal cord, from there to a motor center, and then out along the motor nerves to shoulder, arm, and hand muscles. Trunk and leg muscles respond to support the body in its sudden change of position, and the head and eyes turn to look at the cause of the injury. All this happens while the person is becoming aware of the burning sensation. A reflex that protects the body from injury, as this one does, is called a nociceptive reflex. Sneezing, coughing, and gagging are similar reflexes in response to foreign bodies in the nose and throat, and the wink reflex helps protect the eyes from injury.

A conditioned reflex is one acquired as the result of experience. When an action is done repeatedly the nervous system becomes familiar with the situation and learns to react automatically, and a new reflex is built into the system. Walking, running, and typewriting are examples of activities that require large numbers of complex muscle coordinations that have become automatic.

Nerve pathway of a simple reflex. When the sensory nerve ending is stimulated, a nerve impulse travels along a sensory (afferent) neuron to the spinal cord. Here an association neuron transfers the impulse to a motor (efferent) neuron. The motor neuron carries the impulse to a muscle, which contracts and moves a body part.

abdominal r's contractions of the abdominal muscles about the navel on stimulating the abdominal skin. It indicates that the spinal cord from the eighth to the twelfth thoracic nerve is intact.

cough reflex the sequence of events initiated by the sensitivity of the lining of the airways and mediated by the medulla as a consequence of impulses transmitted by the vagus nerve, resulting in coughing, i.e., the clearing of the passageways of foreign matter.

cremasteric reflex contraction of the ipsilateral cremaster muscle, drawing the testis upward, when the upper inner aspect of the thigh is stroked longitudinally.

deep reflex one elicited by a sharp tap on the appropriate tendon or muscle to induce brief stretch of the muscle.

Magnus and de Kleijn neck r's extension of both limbs on the same side, or one limb or part of a limb, with increase of tonus on the side to which the chin is turned when the head is rotated, and flexion with loss of tonus on the side to which the occiput points; it usually indicates decerebrate rigidity.

Mayer's reflex opposition and adduction of the thumb combined with flexion at the metacarpophalangeal joint and extension at the interphalangeal joint, on downward pressure of the index finger.

Mendel-Bekhterev reflex dorsal flexion of the second to fifth toes on percussion of the dorsum of the foot; in certain organic nervous disorders, plantar flexion occurs.

micturition reflex any of the reflexes necessary for effortless urination and subconscious maintenance of continence.

pilomotor reflex the production of goose flesh on stroking of the skin.

placing reflex flexion followed by extension of the leg when the infant is held erect and the dorsum of the foot is drawn along the under edge of a table top; it is obtainable in the normal infant up to the age of six weeks.

plantar reflex plantar flexion of the foot when the ankle is grasped firmly and the lateral border of the sole is stroked or scratched from the heel toward the toes.

proprioceptive reflex a reflex that is initiated by stimuli arising from some function of the reflex mechanism itself.

psychogalvanic reflex decreased electrical resistance of the body due to emotional or mental agitation.

pupillary reflex

1. contraction of the pupil on exposure of the retina to light.

2. any reflex involving the iris, resulting in change in the size of the pupil, occurring in response to various stimuli, e.g., change in illumination or point of fixation, sudden loud noise, or emotional stimulation.

stepping reflex movements of progression elicited when the infant is held upright and inclined forward with the soles of the feet touching a flat surface; it is obtainable in the normal infant up to the age of six weeks.

stretch reflex reflex contraction of a muscle in response to passive longitudinal stretching.

sucking reflex sucking movements of the lips of an infant elicited by touching the lips or the skin near the mouth.

suck-swallow reflex rhythmical sucking and swallowing movements in an infant when a finger or nipple is placed in the mouth.

superficial reflex any withdrawal reflex elicited by noxious or tactile stimulation of the skin, cornea, or mucous membrane, including the corneal, pharyngeal, and cremasteric reflexes.

tendon reflex contraction of a muscle caused by percussion of its tendon.

tonic neck reflex extension of the upper limb and sometimes the lower limb on the side to which the head is forcibly turned, with flexion of the contralateral limbs; seen normally in the newborn. If it persists into the second or third year of life, it indicates a neurologic disorder.

triceps reflex contraction of the belly of the triceps muscle and slight extension of the upper limb when the tendon of the muscle is tapped directly, with the limb flexed and fully supported and relaxed.

vestibular r's the reflexes for maintaining the position of the eyes and body in relation to changes in orientation of the head.

vestibulo-ocular reflex nystagmus or deviation of the eyes in response to stimulation of the vestibular system by angular acceleration or deceleration or when the caloric test is performed.

vomiting reflex the reflex for vomiting, caused by reflexive stimulation of muscles of the gastrointestinal tract and throat; it is mediated by centers in the medulla oblongata and can be set in motion by a variety of stimuli. See also gag reflex.

relaxation of the anal sphincter which occurs with distention of the rectum and during defecation.

stepping reflex

movements of progression elicited when the animal is held upright and inclined forward with the feet touching a flat surface.

stretch reflex

reflex contraction of a muscle in response to passive longitudinal stretching.

superficial reflex

any withdrawal reflex elicited by noxious or tactile stimulation of the skin, cornea or mucous membrane, including the corneal and pharyngeal reflexes.

tapetal reflex

tapetal reflection of light.

tendon reflex

a method of testing the patency of reflex arc. The tendon is stretched sharply by tapping it close to its insertion. A positive reaction is a sharp contraction of the muscle of which the tendon is part. The patellar reflex is the best known of these reflexes. Absence indicates a defect in the reflex arc, an exaggerated response suggests an upper motor neuron lesion. Called also myotactic reflex.

a test of the ophthalmic and mandibular divisions of the trigeminal nerve, and the abducens nerve. Light touching of the cornea should result in retraction of the globe and protrusion of the nictitating membrane.

reflex walking

spinal reflexes can provide support and uncoordinated use of the hindlegs that resembles walking, in dogs with transection of the spinal cord between T13 and L4.

whisker reflex

pinching the pinna elicits a twitch of the whiskers in cats; used in assessment of depth of anesthesia.

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